This invention relates to a process for removing liquid from thickened sludge by pressing.
Sludge comprising finely divided solids in admixture with a liquid, such as water, is a by-product obtained by many industrial processes. An example of such sludge is lime sludge which is a by-product obtained in the production of sugar.
In may cases the concentration of solids of sludge is relatively low, i.e., from 0.5 to 20% by weight, and in order to permit a satisfactory handling of such sludge it is, therefore, necessary to remove a substantial amount of water therefrom so as to form a thickened sludge.
Various apparatuses are suitable for producing thickened sludge. Examples of such apparatuses are continuous centrifuges, such as decanting centrifuges, belt presses in which the sludge is compressed between two continuously travelling belts and drum filters in which sludge is sucked onto a filter cloth mounted on the exterior surface of a rotating drum by means of a vacuum generated within said drum.
The thickened lime sludge formed by using such apparatuses ordinarily has a concentration of solids of between 15 and 55% by weight. The thickened lime sludge is a plastic mass which is neither liquid nor solid.
However, if subjected to a powerful mechanical treatment, e.g. pumping, the thickened lime sludge may be made flowable and in that condition it may be pumped and transported to sludge dewatering reservoirs. Such sludge containing reservoirs, which occupy large areas, often produce a bad smell because the sluge starts to putrefy during the relatively long periods of time in which the sludge has to remain therein in order to be dewatered.
After storage in such reservoirs, the dewatered lime sludge is removed therefrom and may be utilized as a fertilizer and/or a soil improving material.
It is desirable to remove further amounts of water than possible by storing lime sludge in sludge dewatering reservoirs in order to produce a solid dry product which can be handled in a more convenient manner than the dewatered sludge produced as described above.
It has been attempted to remove water from thickened sludge by pressing, and in a prior art apparatus the thickened sludge is introduced into a cylinder comprising a piston and means for draining off liquid and is compressed therein so as to form a sludge cake.
In practice, however, it has been found that it is very difficult to provide an efficient and stable packing between the piston and the cylinder, i.e., between the high pressure zone and the low pressure zone of such an apparatus.
The main object of the invention is to convert thickened sludge into a low moisture containing material by a process, which does not suffer from the drawbacks of the prior art process.
A further object of the invention is to increase the contration of solids of thickened sludge, e.g., from 15-55% to 20-90% by weight.